We have performed x-ray scattering studies of freely suspended, thick (∼2μ), oriented dimyristoyl-phosphatidyl-choline (DMPC)-water multilamellar films with the emphasis on the ordered phases. These stable films were drawn in situ on a triple axis x-ray spectrometer at a controlled, continuously variable temperature (T) and relative humidity (RH). The structure was determined at several temperatures and humidities and a T-RH phase diagram was constructed. We have found that the previously labeled ordered Lβ' phase is in fact three distinct two-dimensional phases differentiated by the direction of chain tilt with respect to the in-plane lattice. The line shapes of the scattering data indicate that a lower limit of ∼200 Å may be placed on the in-plane chain-chain correlation length, while the chain positions are uncorrelated across the water layers.